1. Field of the Invention
The present invention relates to an arc welding method of the type in which the arc itself causes the movement of a torch to accurately follow the joint to be welded, and more particularly the invention relates to an automatic tracing control method for the arc welding torch.
2. Description of the Prior Art
To realize an unattended automatic welding, it is necessary to use a torch sensor and a torch position adjusting mechanism for automatically sensing and controlling the torch position with respect to the two-dimensional deviation of the groove line which varies from instant to instant during the welding operation.
Various sensors of the above type have been used in the past and they include the contact types, such as, the differential transformer, potentiometer and limit switch types and the non-contact types, such as, the electromagnetic and optical position detecting types. However, since the use of these sensors requires that a sensor or specific device be positioned near the torch in any case, it is necessary to maintain a predetermined distance between the sensing position and the controlled system position due to the dimensional limitation and it has been possible to realize only such practical control systems which have a limited accuracy despite their complicated constractions and in which the torch position is controlled by providing the sensor output with a time differential corresponding to the dimension of the spacing.
Japanese Patent Publication No. SHO 57-3462 invented by H. Nomura and Y. Sugitani and assigned to the same assignee discloses a technique which overcomes the dificiencies of the above-mentioned prior art methods of the type using a sensor. More specifically, in a consumable electrode arc welding or non-consumable electrode arc welding in which the wire feed speed is maintained constant, a torch driving mechanism is provided to compensate the arc characteristics for variations in the wire extension or the arc length and the amount of movement of the torch is always monitored by the mechanism thereby utilizing the axial-direction displacement or position of the torch for tracing control purposes. This method makes it possible to effect the welding stably without the provision of a tracing sensor as a separate member. In other words, in this conventional arc welding method of the arc sensor type, the movement of the torch in its axial direction is controlled by the driving mechanism so that a predetermined arc voltage is maintained if the welding power source is a DC constant current source and a predetermined welding current is maintained if the welding power source is one having a DC constant voltage characteristic. Then, in combination with a torch driving operation which reciprocates the torch in the width direction of the groove of the base metal along the welding direction at a predetermined speed (hereinafter referred as a weaving speed) while moving the torch in the welding direction along the groove line at a predetermined speed (hereinafter referred to as a welding speed), the turning point of the weaving operation is set to each point where the displacement or position of the torch in its axial direction attains a predetermined value thus causing the torch to always weave within the width of the groove parallelly to the groove face and performing a tracing operation and also the height from the base metal surface at the joint to be welded to the terminal end of the weaving is always held constant thereby performing the arc welding. While, in this case, variation of the groove width with respect to the welding direction causes variation of the groove cross-sectional area for each half cycle of the weaving, in accordance with the conventional arc sensor-type arc welding method the axial displacement of the torch for each half cycle is integrated over the time and a signal corresponding to the cross-sectional area of metal deposition is generated for each cycle. Also, separately the change of the distance in the axial direction for each cycle is differentiated to generate a signal corresponding to the distance between the terminal ends of the weaving for each cycle. Thus, by using these signals obtained during the preceding cycle, the preset values of the welding conditions including the welding current, arc voltage, welding speed, etc., are corrected in accordance with the separating determined algorisms and in this way the desired metal deposition as well as the proper penetration bead in the case of the one-side welding are always ensured.